1. Field of Invention
The present invention relates to a method for producing a selective absorption sheet of solar radiation made of stainless steel and having an improved selective absorption property of solar radiation as well as having improved properties in rust proofing, weldability, heat resistance and the like. The present invention is also related to a selective absorption sheet of solar radiation made of stainless steel and produced by the method mentioned above.
The selective absorption sheet of solar radiation is required to have a good selective absorption property of the solar radiation, i.e. a good energy-transmission property within the wave length of solar light and the lowest possible energy emission of solar light having a wave length range corresponding to that of a radiation spectrum of a black body at 100.degree. C. In addition, it is necessary for the practical application of the selective absorption sheet of solar radiation to provide such sheet with good rust proofing and heat resistance, and it is necessary for assembling the pieces of such sheet as a solar collector to provide such sheet with a good weldability.
2. DESCRIPTION OF PRIOR ART
It is well known that material, such as black paint, which closely resembles a complete black body, is applied on the radiation-collecting surface of hot water devices which are practically used for collecting the solar radiation. The radiation-collecting surface for solar radiation, which is provided with black paint, has a good absorption property of solar radiation, but the thermal loss due to emission of thermal energy from the radiation-collecting surface considerably increases as the temperature of the radiation-collecting surface is elevated. The collecting efficiency of a radiation-collecting black painted surface is, therefore, disadvantageously reduced, when the selective absorption surface must operate under such a high temperature as in the solar radiation utilization system. In order to effectively collect the solar radiation, there is a demand to develop such a radiation-collecting surface that not only the collection efficiency should be high but also the emittance of the thermal energy absorbed in the radiation-collecting surface should be low. Radiation-collecting surfaces having a high collecting efficiency of solar radiation and a low emittance of the absorbed thermal energy have been hitherto referred to as the selective absorption surface, and the selective absorption surfaces with copper oxide, plated black nickel or vacuum-evaporated various metal oxides provided on a metal base are known. These selective absorption surfaces, however, involve problems of poor durability and adhesion properties. Their production also involves a number of difficulties in the formation of a uniform plated or vacuum evaporated film. It is particularly essential from the point of view of durability to prevent a decrease in collecting efficiency of the selective absorption surface due to rust formation during its service. The formation of the selective absorption film on a stainless steel sheet, which exhibits excellent corrosion resistance and rust proofing, is very advantageous from the point of view mentioned above.
It is proposed in Japanese Laid Open Patent Application Nos. 52-38652 and 55-6414 that a stainless steel sheet is subjected to a chemical conversion treatment within an alkaline oxidizing bath of a sodium hydroxide or potassium hydroxide solution, in which phosphate of either sodium or potassium, or nitrate or nitrite of either sodium or potassium is incorporated. The film formed in the alkaline oxidizing bath is, however, likely to have a poor uniformity and a so called "irregularity". In addition, since the composition of the alkaline oxidizing bath is complicated, the method of forming the film by means of the alkaline oxidizing bath necessitates an impractical control of the bath.
The method for forming a selective absorption film on a stainless steel sheet proposed in U.S. Pat. No. 3,210,200 is one using as chemical conversion bath an acidic oxidizing bath of a chromic anhydride-sulfuric acid solution. The method and product by means of an acidic oxidizing bath proposed in U.S. Pat. No. 4,097,313 discloses using a solution of sulfuric acid with sodium bichromate or potassium bichromate, in which the concentration of the sodium or potassium bichromate is from 100 to 400 g/l and the concentration of the sulfuric acid is from 400 to 800 g/l. When using these acidic oxidizing baths, a local corrosion, mainly the intergranular corrosion, of the stainless steel blank sheet takes place during the formation of the film, although the degree of corrosion is somewhat varied depending upon the the proportion of sulfuric acid to the chromic anhydride or bichromate. As a result of the local corrosion, the selective absorption property is lowered and proofing against the rust formation due to the presence of chlorine ions, as in the salt spray test, is deteriorated.
Conventionally, in order to assemble the pieces of selective absorption sheet made of metal, such a stainless steel, as a heat collector, the metal sheet pieces cut into a predetermined dimension are shaped and welded to each other. The assembled selective absorption sheet pieces are then provided with a film on the surface thereof. Various improvements for reducing the production cost of the selective absorption sheet are performed, but the surface treatment cost of the conventional method, which accounts for a considerable proportion of the production cost, cannot be reduced to a level lower than a certain limit because of the following reasons. The chemical conversion treatment of the stainless steel sheet for forming the selective absorption film is carried out conventionally by dipping the shaped and assembled sheet pieces into the chemical conversion bath, so that the vessel containing the chemical conversion bath is required to have an extra space for the heat collector as well as a space for the volume of the sheet pieces. In addition, since air cannot be expelled from the interior of the heat collector, the apparent specific gravity of the heat collector is decreased, which leads the dipping operation to be complicated. Furthermore, the chemical conversion solution may be left in the clearances between the assembled sheet pieces after the chemical conversion or coloring treatment, so that rinsing of the heat collector after the chemical conversion treatment becomes difficult.
The coating method of the individual shaped sheet pieces is of a low production efficiency, because the shaped sheet pieces are coated and baked separately.
The problems of the conventional methods could be solved and the production cost of the heat collector could be decreased considerably, if the finished article, i.e. the solar collector, could be manufactured by preliminarily carrying out the coating or chemical conversion of a coil or cut sheets, then shaping and welding the coil or cut sheets with the coating film. Incidentally, most welding methods of the selective absorption sheet pieces presently carried out in practice are based on electric resistance heating, such as in the seam welding and spot welding. And, the selective absorption films presently used in practice are the acryl resin film with the mixed black pigments in the case of the black coating film and is mainly comprised of oxides or hydroxides of chromium or iron in the case of the chemical conversion film described hereinabove. These films are electrically insulating or semiconductive, so that their high electric resistance makes the seam or spot welding based on the electric resistance heating difficult.